1. Field of the Invention
2. Description of the Related Art
The invention relates to a safety contact strip having at least two conducting switching layers in a non-conducting, closed switching chamber within a profile forming an outer jacket.
Safety contact strips are often used as closing edge safety devices at pinching and shearing points. Fitted on gates, machines and handling equipment, persons and material are protected by such safety contact strips. For this purpose, the known safety contact strips are usually held on a respective fastening side in an aluminum carrier profile.
A safety contact strip of the generic type has a contact-maker profile which is held in a carrier profile and in which a separately formed safety contact strip is accommodated in a switching chamber within the outer jacket of the profile. The special shaping of the contact-maker profiles, consisting in particular of an EPDM or an NBR rubber, protects the safety contact strip lying in the switching chamber from damage and permits not only switching loads for switching the contact-maker profile that are perpendicular to the oppositely lying switching layers of the contact-maker profile.
The advantage of the separate formation of a safety contact strip is firstly to be seen in that it can be accommodated in extremely different contact-maker profiles, in which only the switching chamber is to be formed in cross section in a way corresponding to the safety contact strip. Further advantages are offered by the choice of material components with regard to a highly insulating outer jacket of EPDM with two conductive switching layers lying opposite each other on the inside. These switching layers may also consist of an EPDM material, which is often a comparatively poor electrical conductor. Therefore, metallic conductors, for example stranded copper wires or special metal meshes, which bond well in the conducting layers, are often used in these conducting layers to reduce the ohmic resistance.
DE G 93 08 344.0, DE 94 22 030 U1, DE 93 21 338 U1 or EP 0 654 575 disclose safety contact strips of another type, in which a profile is provided with a single hollow chamber, which has on the inside at least two switching layers lying opposite each other. Although in the case of safety contact strips of this type only a low switching load is necessary, there is the great risk of soiling of the switching layers, for example in the event of breakage of the profile. Since these safety contact strips can be produced from a conductive material and a non-conducting EPDM material, which materials can also be coextruded to form these safety contact strips, there is the great risk of the comparatively thin, outer protective layer of the non-conducting EPDM material not being correctly formed and the conductive layer remaining outwardly uninsulated and easily liable to damage.
Against this technical background, the invention has the object of providing a safety contact strip of the type in question of high quality, but at the same time of comparatively simple construction, which switches reliably, in particular even under low switching loads, not only from the direction which is actually to be expected.
According to the invention, the profile, the switching chamber and the switching areas, each with at least one embedded conductor, are formed in one piece. As compared with a two-part formation, with a contact-maker profile and a pushed-in safety contact strip, a one-piece formation is advantageous from technical production-related aspects. In particular, the measures of the one-piece formation also allow the thickness of the walls of the switching chamber to be kept relatively small, without there being the risk of soiling in the event of breakage of the outer jacket. The measure of a thinner wall of the switching chamber also significantly improves the response of the safety contact strip under a switching load. In particular, the omission of a separately formed enclosure of the switching layers produces a significant saving of material.
Alternatively and in particular in combination with the aforementioned features, a better switching performance can also be achieved by an embodiment wherein the switching chamber is kept free in a cross section by webs formed in the manner of spokes. At least two webs, preferably three or four webs, of a material thickness which are often within the range of the wall thickness of the profile in the region of the loads to be expected, hold the switching chamber, the wall of which is made much thinner. In this way, a reliable application of force under a switching load of the outer jacket on the switching layers within the switching chamber is ensured. In particular, this measure permits force to be introduced not only in a preferred direction but additionally also from directions which are significantly inclined onto the switching areas of the switching layers.
In this respect, consideration has been given to the fact that an outer jacket of a profile, webs within a profile and a switching chamber form a plurality of chambers that are separate from one another within the profile.
In a particularly advantageous way, it may be provided that the safety contact strip according to the invention is a coextrudate. This produces a simple, one-piece formation of this safety contact strip, which additionally ensures a reliable function. Obtained as a coextrudate of one or more of the preferred materials, silicon, EPDM, NBR, PVC, TPE and further known, expediently elastic, at the same time resistant materials, is a safety contact strip that is formed in one piece and can be optimally adapted to specified mechanical and electrical requirements by the selection of the materials used. In particular in the case of such a coextrudate, it is preferred that the profile is formed from a TPE, that the switching layers are formed from a conducting EPDM and that the conductors are formed from a metal. Consequently, significantly different materials are used here, to be specific non-conducting, thermoplastic elastomers and ethylene/propylene-diene terpolymers and also incorporated metal conductors, for example copper wires, stranded copper wires, metal meshes or the like, whereby much improved switching characteristics can also be achieved. In particular, the outer jacket of the profile of TPE can be adapted comparatively easily to the degree of switching load. TPE is also much more resistant than many chemical compounds such as are also used, for example, in the automotive industry, where safety contact strips of this type are used for example in the case of electric window lifting mechanisms. It is also possible to form the conducting EPDM switching layers specifically with regard to their ohmic resistance, without having to make allowance for an outer, non-conducting EPDM enclosure.
Alternatively and nevertheless expediently against the background of disposal, the profile and the switching layers may consist of a single-grade plastic and the conductors may consist of a metal. For this purpose, the plastics presented above can be used. In this case, only the electrical conductivity of a plastics material has to be differently set, although this has no bearing on joint disposal. The metal may in this case be disposed of in a way known per se or by means of magnetic separators.
If the material, for example a TPE, of the profile has a Shore hardness of 50 to 70, in particular of 60, this produces a balance between a force of resistance against a switching load and a necessary softness for elastic deformation, the elastic deformation having the effect of exerting a pressure via the webs on the switching chamber, by which the switching layers come for example into touching electrical contact.
In particular, if it is used as the material for the outer jacket of the profile, the TPE material, as a non-conductor, should also have an electrical resistance of over 30 Mxcexa9.
In a preferred embodiment of the safety contact strip according to the invention, a sealing lip attached to the profile is provided. The sealing lip may consist of a separate material or of the material of the jacket of the contact strip. A material which is, as far as possible, smooth on the surface and permanently elastic, to allow it to retain its elasticity for example when water penetrates into pores of the material and when there is frost, has proven to be expedient as a material for this. Such a sealing lip may be molded-on, but it is preferred that the complete safety contact strip with the attached sealing lip is produced as a coextrudate, whereby different materials are processed together. These are the non-conducting profile jacket, the resistance of which is to be greater than 30 Mxcexa9, the conducting switching layers, which are to be formed to be as highly conductive as possible, with an embedded metallic conductor, which may in particular also be formed in the manner of a woven fabric, for particularly good bonding in the material, and also the material for the sealing lip of such a material which for example prevents freezing attachment or has good sliding properties, according to requirements.
If, however, the disposal consideration is uppermost, a safety contact strip with an attached sealing lip may alternatively also be produced from a single-grade plastic.
In a way corresponding to an attached sealing lip of a separate material, the jacket of the profile may be formed at least in certain portions such that it has a separate surface coating and/or the jacket of the profile may consist at least in certain portions of an elastic material, in particular with a smooth surface. Here, too, production as a coextrudate is again envisaged, to also ensure good bonding of the materials to one another. By these measures, the surface property of the jacket of the profile can be set to the necessary requirements, for example with regard to good sliding properties, surface smoothness of the profile and so on.
In a further refinement of the invention, it may be provided that, in a cross section, the safety contact strip is symmetrical in relation to a longitudinal center plane. This longitudinal center plane will then also form the plane of symmetry for the switching load, since the application often requires reliable switching under a switching load at an included angle of about 90xc2x0.
Alternatively, other forms of profile, which satisfy special requirements under specified loads, are entirely possible.
The safety contact strip according to the invention may have a substantially rectangular cross section with rounded longitudinal edges on the side on which the switching load is expected. However, a profile of which the outer jacket has a parabolic cross section, the apex point of the parabola being arranged lying opposite the fastening side, has proven to be much more advantageous.
In the case of a profile that is particularly of such a parabolic form, in a refinement of the invention it is preferred for the sealing lip to be tangentially attached. Projecting edges are avoided as a result, and the introduction of force from the sealing lip into the profile is usually favorable. This avoids the sealing lip being easily tom or even torn off, for example in the event of freezing attachment or jamming. It has also proven to be expedient, in particular in the case of scaling lips which are attached at less curved portions of the jacket profile, for the free end of the sealing lips to be provided in a cross section with a bend. On the basis of this measure it is possible to form a sealing lip of which the free end covers for example the apex point of the parabolic profile. Alternatively, the sealing lip may also be given a bend toward the outside, if appropriate protruding beyond the profile.
In particular in the case of such a parabolic cross section of the jacket, it has also proven to be expedient that, opposite the fastening side of the profile, two webs formed in a V-shaped manner hold the switching chamber, the angle bisector of the. V-shaped opening of the two webs lying in particular in the plane of symmetry of the profile. Every switching load which occurs on the profile within the V-shaped opening will reliably lead to activation of the safety contact strip according to the invention, and so too will those switching loads which occur significantly outside this V-shaped opening.
The reliability of the activation of the safety contact strip according to the invention under directionally independent switching loads is further enhanced by the measure that, on the fastening side of the profile, at least one web holds the switching chamber, at a distance from a base of the profile. If a central web is provided, this measure allows the switching chamber to tilt freely within the profile and adjust itself in such a way that a relatively small elastic deformation of the profile, even one from the side, leads to reliable activation of the switching contact.
Alternatively, two webs, if appropriate more than two webs, may also be provided for this purpose, consideration then also being given to a symmetrical arrangement of the same. These webs also preferably have a material thickness approximately the same as that of the other webs.
According to a further feature of the invention, the jacket is provided with longitudinally running grooves, in particular on the inner side. These grooves specifically weaken the material strength of the jacket wall, so that a buckling-in, folding-in as it were, of the profile can occur in such regions, or at least easier elastic deformation of the jacket. These measures also make further allowance for a reliable switching performance and, at the same time, such longitudinal grooves may be useful for the extrusion operation.
Comparably to previous arrangements, it may be provided that the switching chamber has an elliptical cross section, the longitudinal axis of the cross section of the switching chamber in particular being perpendicular to the plane of symmetry of the profile. The conducting switching layers are then arranged above and below this longitudinal axis.
It may be expedient in the case of these conducting switching layers if they have surfaces of a concave form. This reduces in a cross section the distance of these conducting layers from the outer edges, which means a smaller switching path.
If a switching chamber of elliptical cross section is provided, a concave-convex cross section is then expediently to be provided for the conducting layers.
Critical regions of safety contact strips quite generally are the points at which the conducting layers are provided with connecting cables. Separate contacting is preferred, since safety contact strips can then be provided in the form of endless profiles.
Alternatively, the embedded metallic conductors may for example also undertake the function of connecting cables, if these conductors are correspondingly led out during production. It can be regarded as disadvantageous in this respect that only made-up safety contact strips of a specific length are then available.
If separate contacting is provided, this often takes place on the end face of the safety contact strip. In this region, reliable operability of the contact strip is often no longer obtained, since a deformation of the switching chamber often cannot take place. This effect is mitigated in particular also in the case of safety contact strips according to the invention if, at the end face, in the region of this electrical contacting of the conducting layers, this region of the switching chamber is made free of forces by cuts. As a consequence, a deformation of the profile will take place in such a way that a buckling effect occurs in the region of the cut ends, whereby the switching contact is then likewise brought about by touching of the conductive switching layers. In particular, consideration can also be given to making the switching chamber free of forces by cuts in the webs.
Allowance is further made for the high standard of the safety contact strip according to the invention by providing a cap for closing the end face of the safety contact strip. The cap is designed in such a way that, adapted to the profile of the safety contact strip, a shoulder running around annularly in a plane is provided for abutment of the end face of the jacket and of the base of the same. In particular, such an annularly running-around shoulder is suitable for sealed adhesive bonding and/or fusing with the end face of the jacket or of the base. Less consideration is given to adhesive bonding of the end faces of the webs, to continue to permit satisfactory switching. In order to ensure this, it is further provided that the material of the cap has a hardness of less than 65 Shore, in particular of less than 60 Shore, and consequently is significantly softer than the other material chosen. It helps to accomplish this if the material thickness of the cap is kept comparatively small.
In the case of a preferred embodiment, it is provided that lips projecting in front of the shoulder for engagement of the outer jacket. In this respect, it is envisaged in particular that an outer lip covers the abutting edge against the outer jacket of the profile and, if appropriate, is in turn adhesively bonded or fused with the latter. This produces very reliable sealing of the ends at the end faces of the safety contact strip according to the invention. Lips projecting within the profile for an inner engagement of the outer jacket have to make allowance in their formation for the webs of the profile and, if appropriate, be interrupted, and are preferably formed as simple projecting bar-like portions.
For particularly good abutment, in particular also of the outer lip on the outer side of the outer jacket, it is further provided in a refinement of the cap according to the invention that the engagement of the jacket causes it to undergo prestressing, producing particularly good abutment of the outer lip of the cap and of the surface of the jacket.
In a further refinement of the cap according to the invention, it may be provided that at least one sealing lip is provided on the upper side of said cap. This sealing lip is often formed such that it is somewhat smaller than the sealing lip of a safety contact strip. In particular, the sealing lip associated with the cap is arranged within a radius of curvature of the sealing lip of the safety contact strip. As a result of this measure, provided that the sealing lip of the cap attached to a safety contact strip butts against a sealing lip of the safety contact strip, when the sealing lip of the safety contact strip is pressed down this movement is also reliably transferred to the cap. A measure which is useful for the switching performance at the end of a safety contact strip.
The invention is explained in more detail on the basis of the drawing, in which merely exemplary embodiments are represented.